<PAGE>
            COLLABORATIVE ACTIVITIES AT LABORATORIES IN MACCHIAREDDU

     1.   Within the newly leased facility (total 850 sq. m), 250 sq. m of space
          (ex Stabulario) will be exclusively made available for use by Idenix
          employees. Idenix employees will also enjoy of common area (total 350
          sq. m) use similar to all other University employees occupying the
          building. Idenix will contribute up to E 54,000 per year during 3
          years to the cost of University of Cagliari for renting this facility
          subject to the conditions noted in paragraph 4 below.

     2.   Idenix will restore and equip at its own expenses its laboratory space
          at Macchiareddu, that will include a BL3 laboratory.

          a.   Idenix will have priority for use of the BL3.

          b.   Pr. LaColla will be administratively responsible for the BL3
               physical plant.

          c.   Idenix equipment at Macchiareddu will become property of the
               University of Cagliari when the Cooperative Agreement between the
               University and Idenix expires or is terminated.

     3.   Pr. LaColla will have access to all Idenix compounds and scientific
          data generated as a follow up of cooperative activities previously
          involving the Cagliari Team and will be named as a co-inventor on any
          Intellectual Property to which he has contributed concerned with the
          developments of such research activities. Pr. LaColla will be named as
          a co-inventor on Intellectual Property, for which he meets the
          criteria as a contributor to the invention, that will be generated by
          the Idenix team in Cagliari or the Cagliari team.

     4.   Pr. LaColla will work with the Director of Virology (see below) to
          establish the budget of the University of Cagliari - Idenix
          Cooperative Laboratory (Cooperative Laboratory). Idenix will provide
          the following (in 1000s of Euro) for the Cooperative Laboratory. As
          set forth below, the total 2004 budget for the Cooperative Laboratory
          will be 1.363MM Euro.

<Table>
<Caption>
                                             Idenix         UNICA
                                             ------         -----
<S>                                          <C>            <C>
Running Cost                                 275
Salaries and bonus pool                      560
Travel                                       20
Capex (to be agreed between Pr La Colla
   and D. Shlaes)                            208
Rent (Macchiareddu)                          54
Unrestricted Grant                                          250
</Table>

          CAPEX will be agreed between Pr. LaColla and D. Shlaes.

          Idenix will provide the University of Cagliari with an unrestricted
          grant of E 250,000 per year during 3 years [2004 - 2006], which can be
          used for travel, salaries, running costs, etc. Idenix will pay the
          unrestricted grant on a monthly basis. However, the obligation of
          Idenix to make such payment and other payments relating to the
          operation of the Cooperative Laboratory is conditioned upon the
          Co-operative Antiviral Research Activity Agreement dated January 4,
          1999, by and between Dipartimento di Biologia Sperimentale "Bernardo
          Loddo" dell Universita di Cagliari and Idenix SARL, as amended,
          remaining in full force and effect throughout such period.

     5.   Pr. LaColla will remain the Director of the Cooperative Laboratory and
          will continue to provide scientific guidance to the Idenix team.

     6.   Idenix has identified and employ a senior Director of Virology for the
          Idenix Team in Cagliari. The roles of the Director of Virology vis a
          vis Idenix, the Cooperative Laboratory and Pr. LaColla are described
          below.

GENERAL MISSION

In accordance with the Idenix' R&D objectives defined by the Exec V.P. for R&D,
Director, Virology, in concert with the Cooperative Laboratory Director, will
propose and implement Idenix' strategy for anti-viral screening within the
facilities at the University of Cagliari utilized by the Idenix employees
and/or through academic collaborations or contract organizations.

Director, Virology, in concert with the Cooperative Laboratory Director, will
be responsible for budget proposal and implementation of the approved budget
for the Idenix team in Cagliari (head count, scientific equipment, consumables,
travel, training, scientific consultancies).

Director, Virology will be responsible for Idenix Human Resources management
concerning yearly appraisal, salary and rewarding proposal, recruitment
proposal and selection.

CONTEXT AND STAKES OF THE POSITION

Director, Virology will be based in Cagliari within Idenix SARL.

Internal relations
------------------

Director,, Virology reports to the Executive V.P. for R&D, who is located in US
within Idenix Pharmaceuticals Inc. (Mass). For all budget and HR questions in
EU, he follows Idenix SARL procedures.

External relations
------------------

Director, Virology will be responsible for the supervision of the Idenix
facilities located on University of Cagliari premises. Supervision will be
assumed in respect of the input and role of the Cooperative Laboratory Director
as defined in the collaborative convention signed with the University of
Cagliari.

The Cooperative Laboratory Director is responsible of the implementation of the
research program and of the budget and means allocated to the Cooperative
Laboratory by the two parties, as defined once a year during the steering
committee meeting. He is responsible for obtaining agreement from the two
parties before any publication and communication. He has a role of proposition
for matters related to rewarding proposal, travel and training for Cooperative
Laboratory personnel. He is responsible for implementing hygiene and security
procedures towards the lab personnel in accordance with the University policy.

Director, Virology will supervise for the benefit of Idenix the implementation
of the research program and of the budget and means allocated by Idenix to the
Idenix Team in the Cooperative Laboratory. This will be done in respect of the
global objectives agreed on by the two parties once a year during the steering
committee meeting. In addition Director, Virology, will be directly responsible
for Human Resources management of Idenix personnel concerning commitment to
Idenex objectives, yearly appraisal, salary and rewarding proposal, travel and
training.

In case of need to modify in a significant way the research program or the
means allocated by Idenix to the Cooperative Laboratory, the Director, Virology
will inform the Idenix members of the steering committee and the President of
the steering committee, the latter may call for an extraordinary meeting for
redefining with the two parties the cooperative lab research program.


--------------------------------------------------------------------------------



Heretofore agreed this 22 day of March, 2004:



/s/ Jean-Pierre Sommadossi                  /s/ Paolo LaColla
------------------------------------        -------------------------
Jean-Pierre Sommadossi, Ph.D.               Prof. Paolo LaColla
Gerant                                      Dep. of Biomedical Sciences and
Idenix SARL                                 Technologies, University of Cagliari
Montpellier, France                         Cagliari, Sardinia, IT
<PAGE>
Cagliari, June 30, 2004

Universita degli Studi di Cagliari
Cittadella Universitaria
SS 554 Km 4,5
09133 Monserrato (CA)

To the attention of Magnifico Professor Pasquale Mistretta

Re:  Collaborative Activities at Laboratories in Macchiareddu


Magnifico Rettore,

Relating to the agreement "Collaborative Activities at Laboratories in
Macchiareddu" dated March 22, 2004 it is understood that there will be a partial
modification to article 4 of the agreement (Rent Macchiareddu) that provides for
Idenix to be responsible for rent in an annual amount of 90,000 euro + tax


Cordaialment

IDENIX SARL

Jean-Pierre Sommadossi

/s/ Jean-Pierre Sommadossi

Agreed


Universita degli Studia di Cagliari

Magnifico Rettore
Prof. Pasquale Mistretta

/s/ Prof. Pasquale Mistretta